How many 12-year-olds go out with a bodyguard? Will Smith‘s mega-achieving son Jaden, star of “The Karate Kid,” showed up to Disney star Jake T. Austin‘s 16th-birthday party Saturday protected by security, a source tells us. He joined other young stars at yet-to-open adult playhouse Beacher’s Madhouse at Hollywood’s Roosevelt Hotel. Guests at the Onitsuka Tiger-sponsored bash included Selena Gomez, David Henrie, Kyle Massey, Chris Massey, Chelsea Staub, Jennifer Stone and Boo Boo Stewart. A source said, “Jaden won a dance-off with another kid. When the girls asked him to go out after the party he said, ‘I’m only 12, I’ve got to go home to my parents.’ “